Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the conduit and the material contained therein.
It is well known to use vibrating flowmeters to measure mass flow and other properties of materials flowing through a pipeline. For example, vibrating Coriolis flowmeters are disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. and also Re. 31,450 to J. E. Smith. These flowmeters have one or more fluid tubes (or “flow tubes”). Each flow tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional, radial, lateral, or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural modes. The vibration modes are generally affected by the combined mass, stiffness, and damping characteristics of the flow tube and the material contained therein, thus mass, stiffness, and damping are typically determined during an initial calibration of the flowmeter using well-known techniques.
Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter. The material is then directed through the flow tube or flow tubes and exits the flowmeter to a pipeline connected on the outlet side.
A driver, such as a voice-coil style driver, applies a force to the one or more flow tubes. The force causes the one or more flow tubes to oscillate. When there is no material flowing through the flowmeter, all points along a flow tube oscillate with an identical phase. As a material begins to flow through the flow tubes, Coriolis accelerations cause each point along the flow tubes to have a different phase with respect to other points along the flow tubes. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors are typically placed at two different points on the flow tube to produce sinusoidal signals representative of the motion of the flow tube at the two points. A phase difference of the two signals received from the sensors is calculated in units of time.
The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes. The mass flow rate of the material is determined by multiplying the phase difference by a flow calibration factor. The flow calibration factor is dependent upon material properties and cross-sectional properties of the flow tube. One of the major characteristics of the flow tube that affects the flow calibration factor is the flow tube's stiffness. Prior to installation of the flowmeter into a pipeline, the flow calibration factor is determined by a calibration process. In the calibration process, a fluid is passed through the flow tube at a given flow rate and the proportion between the phase difference and the flow rate is calculated. The flow tube's stiffness and damping characteristics are also determined during the calibration process as is generally known in the art.
One advantage of a Coriolis flowmeter is that the accuracy of the measured mass flow rate is largely not affected by wear of moving components in the flowmeter, as there are no moving components in the vibrating flow tube. The flow rate is determined by multiplying the phase difference between two points on the flow tube and the flow calibration factor. The only input is the sinusoidal signals from the sensors indicating the oscillation of two points on the flow tube. The phase difference is calculated from the sinusoidal signals. Since the flow calibration factor is proportional to the material and cross-sectional properties of the flow tube, the phase difference measurement and the flow calibration factor are not affected by wear of moving components in the flowmeter.
A typical Coriolis mass flowmeter includes one or more transducers (or pickoff sensors, or simply “pickoffs”), which are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the driver. The pickoffs are connected to electronic instrumentation. The instrumentation receives signals from the two pickoffs and processes the signals in order to derive a mass flow rate measurement, among other things.
Typical Coriolis flowmeters measure flow and/or density through the use of a coil and magnet as a pickoff to measure the motion of a meter's vibrating flow tube/tubes. The mass flow rate through the meter is determined from the phase difference between multiple pickoff signals located near the inlet and outlet of the meter's flow tubes. However, it is possible to measure flow using strain gages in place of coil/magnet pickoffs. A fundamental difference between the two sensor types is that coil/magnet pickoffs measure the velocity of the flow tubes and strain gages measure the strain of the flow tubes which is proportional to the tubes' displacement. As such, the placement of each type of sensor will not necessarily be in the same location.
Strain gages have a number of advantages over coil/magnet pickoffs. Strain gages are cheaper to produce and implement than coil/magnet pickoffs. They also help to eliminate point masses that may adversely affect system operation. Additionally, strain gages do not need a reference point from where to measure strain like coil/magnet pickoffs. This allows for single flow tube designs that are not possible with coil/magnet pickoffs.
Momentum conservation, according to the conservation of momentum principle, requires that the momentum over a given time remain unchanged as steady flow occurs through an isolated system of fluid, such as through the flow tube of a vibratory flowmeter. Since momentum is a vector quantity, a change in direction of the flow causes a reduction of momentum in the original direction which is offset by an increase in the new direction. Fluid travelling through a bend in a pipe, for example, exerts a force on the pipe which must be counteracted by an anchor force to prevent the pipe from moving. This is the reason thrust blocks are often installed proximate pipe bends in municipal water pipe systems, for example.
In the case of a U-bend, such as is often found in the flow tubes of vibratory flowmeters, the fluid that enters the flow tubes is redirected 180° so return flow travels back in the same direction from which the fluid entered the flow tubes. This change in direction causes the flow to exert two axial y-direction forces on the flow tube: internal pressure and a momentum re-direction.
The embodiments described below provide means to measure fluid momentum. It is an object to provide an embodiment for the measurement of fluid momentum in a pipeline. It is an object to provide an embodiment for the measurement of fluid momentum in a vibratory meter. It is an object to provide an embodiment for the measurement of fluid momentum to detect pipe coating or plugging in a pipeline. It is an object to provide an embodiment for the detection of pipe coating or plugging in a vibratory meter. It is an object to calculate mass and volume flow rate in a vibrating meter using the measurement of fluid momentum.